Understanding of the spectrum of the HIV and HCV epidemics in injection drug users (IDU) has been advanced by a number of outstanding prospective studies conducted in various part of the world, including, North America, Europe, and Australia. Each of these studies has furthered our understanding of disease prevalence and incidence, risk behaviors, and natural history, including clinical, immunological and virological factors. This application seeks funding for the first global collaboration of prospective studies of HIV and HCV in IDU. The International Collaboration of Incident HIV and HCV in Injecting Cohorts (InC3), is merged international multi-cohort project of pooled biological and behavioral data from nine prospective cohorts of IDU. The InC3 collaboration will facilitate new in-depth studies of HIV and HCV infection not possible from each individual study. This collaboration will allow us to study 4,091 IDU who have been followed for a collective 9,016 person-years of observation, 859 incident HCV infections with longitudinal follow up, and 575 HIV infections. This proposal includes six research questions that represent some, but not all, of the potential breadth and depth of the proposed InC3 research collaboration, including: assessment of temporal trends in HIV and HCV incidence, examination of HCV incidence by HIV status and sexual behavior, estimates of rates and determinants of HCV viral clearance and reinfection, examination of the effects of HCV infection disclosure are on risk behaviors, and an evaluation of outcomes related to clinical treatment of acute HCV in IDU. Many of these questions remain unanswered due to the relatively small number of small numbers of behavioral, viral, or clinical events in each study. However, since many have been meticulously characterized in each of these rich longitudinal cohorts, merging data will offer the statistical power needed to make firm conclusions. The significant and innovative scientific agenda and the InC3 leadership will be supported by an administrative core and Data Coordinating Center (DCC). InC3 will promote and support the development of new study questions and research opportunities through supporting research exchange and training for participating junior scientists. Lastly, some gaps in biological data will be addressed with support for laboratory measures from stored specimens. This proposed InC3 study, will accomplish the following: (1) creation of a rich new data source that will fill scientific knowledge gaps that heretofore have not been possible in single studies alone; (2) enable new research relevant to understanding, preventing, and treating HIV and HCV in IDU; (3) the creation of a multilateral collaborative of researchers and scientists committed to long term cooperation and partnership in areas including: epidemiology, behavioral studies, clinical research, and laboratory science. Many of the investigators of this proposed multicenter project are experienced and successful in pooling and analyzing large epidemiological data in both a national, and international, context. Despite the obvious benefits to public health, such an organized effort has never been attempted with cohorts that have studied both HIV and HCV. InC3 is significant, innovative, will advance research relevant to the global burden, consequences, prevention, and treatment of HIV and HCV infections among IDU.